The present invention relates to a broadband multielement antenna of the monopole type, i.e. one having end excitation, which has relatively small bulk.
An antenna of this type consists essentially of rodlike elements (wire, metal rod or the like) through which a current is passed and which is generally known as a "whip." The application of the theory of lines to antennas shows that the impedance of the rodlike radiating element as seen from the transmitter (or the receiver) is a function of the ratio between the geometrical length of the element and the wavelength of the radiated energy. The optimum energy coupling conditions between the transmitter (or the receiver) and the radiating elements are those with which the impedance of the element is entirely real and generally in the neighbourhood of 50 ohms (i.e. a geometrical length of about .lambda./4 in the case of a monopole). These well-known results assume a sinusoidal distribution of the current in the element. It is deduced that:
1. A RADIATING ROD HAS A SELECTIVE IMPEDANCE WHICH VARIES RAPIDLY WITH THE FREQUENCY, AND
2. THE GEOMETRICAL LENGTH OF THE ELEMENTS IS OF THE SAME ORDER OF MAGNITUDE AS THE WAVELENGTH (IN VACUUM, IN THE CASE OF NON-MAGNETIC METALS).
Under these conditions, the problem of the construction of broadband antennas having a limited bulk in the decametric and/or metric bands is particularly complex. It is usual to design antennas operating in these frequency ranges with a conductive cylinder, sometimes hollow, the diameter of which is not negligible as compared with the wavelength (i.e. of a diameter &gt;.lambda./10, where .lambda. is the wavelength in vacuum), the said cylinder often being produced from an assembly of rodlike elements distributed along the directrices of the surface of the said cylinder and sufficiently close together to ensure electrical continuity of the surface. Calculation of the distribution of current in such an antenna is a very complex problem. As a first approximation, it may be compared to a set of coupled rodlike radiating elements. It is known that thickening of the antenna increases the bandwidth. Tuning in the range is obtained with the aid of impedances of appropriate value which are manually or automatically connected in series with the antenna.